Optical systems useable with, for example, light sensitive receivers and/or sensors are known. For example, U.S. Pat. No. 6,560,037 discloses a single element lens with an aperture stop adjacent to a distal surface of the lens element. However, a single homogenous lens element such as that disclosed by U.S. Pat. No. 6,560,037 does not affect the degree of optical imaging correction required to produce the best resolution on sensors employing hundreds of thousands of pixels.
Optical systems having two lens groups or lens elements are also known. For example, two lens group or element reverse telephoto designs are disclosed in U.S. Pat. Nos. 5,677,798; 5,812,327; 6,181,477; 6,097,551; and US patent application US 2002/0018303. A typical reverse-telephoto lens configuration can be described as one where the first lens group or element is negative in power and the second lens group or element is positive in power, when proceeding sequentially from the most distal end of the lens to the sensor. This arrangement causes the back focus of the lens system to be longer than the focal length. The reverse-telephoto configuration is a design form that allows for a generous space between the sensor and the lens element most proximal to the sensor; this space is often used to position additional optics like infrared rejecting filters or a sensor protective plate. Additionally, the reverse-telephoto lens configuration may help reduce the magnitude of the oblique ray angles incident on the sensor.
Other two lens element designs are disclosed that are not of the aforementioned reverse-telephoto configuration. U.S. Pat. Nos. 5,251,069; 6,515,809; US patent application 2003/0016452; US patent application 2003/0048549 are such examples of non reverse-telephoto design forms comprised of two groups/elements. Although these latter design forms may not possess the same typical advantageous configuration of a reverse-telephoto lens, they emphasize other aspects of improvement like enhanced manufacturability, reduced cost or more compact size. In optical systems where there is little need for a space behind the lens for a thick plate or a stringent requirement on limiting the angle at which the chief ray is incident upon a sensor, the non reverse-telephoto design forms may be preferable. However, there is a limitation with any two group/element design form; image quality will generally be less than what it could be with three or more lens groups/elements. For compact lenses designed for miniature sensors with typically a million pixels or more, designs of three or more lens elements may be desirable to achieve a suitably high image resolution.
Optical systems having at least three lens elements are also known. U.S. Pat. Nos. 6,441,971 B2; 6,282,033; 6,414,802; 6,476,982; and JP2002162561 disclose three and four lens element/group systems designed for imaging with sensors, generally with an aperture stop at or near the most distal surface of the lens from the sensor.
Designs like those disclosed in U.S. Pat. Nos. 6,282,033, 6,414,802, 6,476,982, and JP2002162561 are generally of the type comprised of four or more discrete lens elements that are assembled into at least three groups with an aperture of F/4 or less. Although excellent image quality may often be obtained with a sufficient number of optical surfaces, designs like these are expensive because of the number of lens elements that need to be manufactured and assembled versus designs with fewer elements. Furthermore, for a given application, it is difficult to achieve a short overall lens length when more lens elements are used. A useful figure-of-merit in comparing the shortness of lens designs is the ratio of the overall system length L, from the most distal vertex to the image plane, to the effective focal length of the lens f0. Applying this metric, U.S. Pat. No. 6,282,033 discloses a lens with an overall system length of about 8 mm and a focal length of about 4.5 mm. The ratio of the overall system length to focal length for this lens is then a little less than 2. Furthermore, the preferred embodiment for U.S. Pat. No. 6,282,033 is one comprising lenses all made from glass with spherical surfaces. The expense of making all elements from glass exceeds the cost of making elements from resin materials in high volume production. Designs like those disclosed in U.S. Pat. No. 6,476,982 and JP2002162561 consider hybrid glass-plastic forms with the use of aspheric surfaces for aberration correction on the plastic lens elements. JP2002162561 discloses lenses with focal lengths of approximately 5.6 mm and with overall system lengths a little greater than 10 mm; consequently, the ratio of overall system length to focal length is a little less than 2. U.S. Pat. No. 6,476,982 discloses lenses with focal lengths of approximately 5.7 mm and with overall system lengths of approximately 7.35 mm, consequently, the ratio of overall system length to focal length is approximately 1.3. Designs like those disclosed U.S. Pat. No. 6,414,802 B1 are comprised of all plastic elements. The overall system lengths of disclosed lenses are as low as 15 mm for a focal length of 10 mm; consequently, the ratio of overall system length to focal length is approximately 1.5. These lenses are deficient in the area of minimizing the ratio of overall system length to focal length partly because of the use of many lens elements and the inability to compress the design into short lengths.
Other lens designs, like those disclosed in U.S. Pat. No. 6,441,971, are comprised of just three lens elements and have a relatively high light-collecting aperture of F/2.8 and wide field of view. The use of just three lens elements facilitates a more compact design than the lenses employing more elements. The designs disclosed in U.S. Pat. No. 6,441,971 have a ratio of overall system length to focal length of approximately 1.25. Achieving this relatively short length is partly achieved by choice of lens materials. The disclosed designs utilize a positive power glass lens element in the most distal position from the sensor with index of refraction greater than those typical for most plastics and many glasses. For example, SK16 with Nd=1.62041 or C-ZLAF2 with Nd=1.80279 are used in the disclosed designs, whereas most common plastics like acrylic and polystyrene have Nd<1.6. Additionally, the least expensive glass material type, BK7, has Nd=1.517. The refractive power of an air-material interface is greater when the index of refraction is higher and facilitates the design of a more compact system because lens thicknesses can generally be reduced; however, this design advantage comes at the expense of fabrication and material cost for the glass element(s). Designs that use the less expensive plastic resins with Nd<1.6 (e.g., acrylic, polycarbonate) or that include one lens element using the least expensive type glass (e.g. BK7) will generally have a lower overall cost.
The designs disclosed in U.S. Pat. No. 6,441,971 are disadvantaged in that they do not solve a problem that is important to the function of many sensors; namely, a significant reduction in the angles of rays incident upon the sensor. These designs have angles that exceed 20 degrees. It is important to reduce the angles of rays incident upon the sensor for several reasons. One is that it helps to improve the illumination uniformity on the sensor such that the corners are illuminated adequately relative to the center. Additionally, any dichroic-type filters (such as some infrared light rejection filters) will have properties that change versus angle of incidence; it is desirable to minimize these differences. Also, oblique rays incident on a sensor with light-collecting lenses (i.e., a microlenslet array) can become a problem if light associated with one lenslet over a pixel becomes imaged onto an adjacent pixel.
To overcome these issues, it is generally desirable to have all ray angles incident upon the sensor as small as possible. This requirement is often simplified to a constraint on the maximum chief ray angle. The chief ray angle is set by the lens exit pupil location. Lenses with exit pupil locations approaching infinity provide chief ray angles approaching zero, and are commonly referred to as telecentric. Practically, lenses with a high degree of telecentricity tend to come with increased complexity and increased overall length. As these are both key drivers in many imaging applications (e.g., camera-enabled phones and PDAs), some compromise must be made between them and the degree of telecentricty for a given lens solution (while preferably keeping the maximum chief ray angle less than about 20 degrees).